1. Technical Field of the Invention
This invention relates to a pressure control apparatus for hydraulic clutches in a working vehicle such as a tractor, and to a method of executing such control.
2. Prior Art of the Invention
To perform backward and forward drive switching efficiently, a working vehicle such as a tractor provides the same number of speeds for forward drive and backward drive, and includes a hydraulically operable backward and forward drive switching device disposed in an upstream position of a transmission in a propelling drive transmitting line extending from an engine, so that drive switching may be carried out frequently.
Such a backward and forward drive switching device has a large hydraulic capacity, and breaks the propelling drive transmission by disengaging backward and forward drive hydraulic clutches with respect to both backward and forward sides thereof Thus, the drive switching device is assigned to the functions of both a main clutch and an inching clutch. The drive switching device acting to perform such functions dispenses with the main clutch between the engine and the transmission.
Consequently, the hydraulic clutches of the backward and forward drive switching device are controllable by operating commands transmitted through two lines, one of which is given by a switching operation of a shuttle lever for backward and forward drive switching, and the other by depression of a clutch pedal for making and breaking the propelling drive transmission.
The control by the shuttle lever is intended to effect a smooth drive switching. Specifically, two microswitches are provided for electrically detecting forward and backward positions of the lever, respectively. From the time one of the microswitches for detecting the forward and backward positions is turned on, the pressure for the hydraulic clutch corresponding to the selected direction is gradually increased for a fixed time.
The above control is called a modulated pressure control. In the modulated pressure control, a microcomputer controls a current applied to an electromagnetic proportional pressure reducing valve. This renders a pressure increase pattern of pressure oil supplied to the hydraulic clutch optimal for drive switching.
On the other hand, the control by the clutch pedal is based on a clutch pedal sensor for detecting a clutch pedal position (i.e. a position to which the pedal is raised for clutch engagement after being depressed for clutch disengagement). In response to the pedal position detected, the electromagnetic proportional pressure reducing valve is operated to control the pressure for the selected hydraulic clutch. This construction provides the operator with the same operational feeling as when operating an ordinary mechanical clutch.
The pressure increase pattern provided by operation of the clutch pedal is set for each speed by the microcomputer. The pedal is operable with the same feeling for selecting any speed.
In the above conventional backward and forward drive switching device, the backward and forward drive hydraulic clutches are used both as shuttle clutches (backward and forward drive clutches) and a main clutch (drive connecting and disconnecting clutch). Thus, it is necessary to select the pressure instructed by operation of the shuttle lever or the pressure instructed by operation of the clutch pedal, and apply a current as instructed to the pressure regulating valve based on the pressure selected.
Preferably, this selection is made with priority given to the instruction or command occurring later, by taking the inching control of the clutch pedal (i.e. a quick engaging operation repeating a switching between clutch disengagement and half clutching) into account.
However, where the quick engaging operation by the clutch pedal is given priority, an operation of the shuttle lever for an opposite traveling direction (from forward to neutral and to backward, or from backward to neutral and to forward) results in a pressure increase for a different one of the clutches. This would generate shocks to impair a smooth backward and forward drive switching.
A primary object of this invention is to provide a pressure control apparatus and method for controlling a hydraulic clutch device which overcome the disadvantage noted above.